Molten carbonate fuel cell matrix tape and assembly method

ABSTRACT

A matrix material for a molten carbonate fuel cell is described which is flexible, pliable and has rubber-like compliance at room temperature. The matrix has three components comprising fine inert particulate material, larger crack attenuating ceramic particulate material, and an organic polymeric binder. A process of assembling a molten carbonate fuel cell utilizing the compliant matrix material is also described. The compliant matrix material is inserted into a molten carbonate fuel cell stack utilizing fuel cell anodes with sufficient porosity to contain excess electrolyte. The fuel cell is heated up to a temperature sufficient to remove the polymer binder and cause a portion of the electrolyte material present in the anode to wick into the matrix to substantially fill it completely.

This is a division of application Ser. No. 307,123 filed on Sept. 30,1981.

TECHNICAL FIELD

The field of art to which this invention pertains is molten carbonatefuel cells and particularly matrix components for such fuel cells.

BACKGROUND ART

Molten carbonate fuel cells are well known in the art and are described,for example, in U.S. Pat. Nos. 4,009,321 and 4,079,171. The electrolytein this type of cell is solid at room temperatures and is a moltenliquid at operating temperatures which generally range between 500° C.and 750° C. Some well known electrolytes of this type are the alkalimetal carbonate compositions such as ternary lithium-potassium-sodiumcarbonate compositions and binary lithium-potassium, lithium-sodium, orpotassium-sodium carbonate compositions. The electrolyte is disposedwithin a substantially inert matrix sandwiched between an anode and acathode electrode. In addition to having structural integrity, thematrix, in combination with the electrolyte, must provide completeseparation of the fuel and oxidant gases disposed on opposite sidesthereof. The electrolyte and matrix combination is often referred to asan electrolyte tile. The matrix is usually made from submicron ceramicparticles which are compatible with the fuel cell environment. Forexample, lithium aluminate is substantially inert to the ternary andbinary carbonate compositions mentioned above, and may be used as thematrix material in cells incorporating those types of electrolytes.

Typically, such tiles are produced by compression molding the inertmaterial in admixture with the alkali metal carbonates. This method ofproducing the matrix structure suffers from many disadvantages.Compression molding is a relatively expensive forming method requiringrelatively large amounts of time, energy and capital investment. Theresultant molded tile is a relatively thick, fragile ceramic sheet.Accordingly, it is subject to cracking, and great care must be taken inthe design of the fuel cell to provide a flat surface for such sheet toinsure minimal flexural and compressive forces on the tile until heatedabove its melt point.

The poor handleability and critical tolerance requirements dictated bythe use of this type of a matrix structure make scale-up to commercialsizes and quantities unattractive. In addition, a life-limiting,functional problem exists with the compression molded tiles of thistype. As the cell runs, electrolyte is consumed by corrosive reactions,vaporization, and surface migration. In a typical tile cell, theelectrolyte is withdrawn from the larger pores of the matrix. Thelithium aluminate cannot be sufficiently close-packed in a tile toachieve a small, uniform pore size at operating temperature bycompression molding. Therefore, electrolyte withdrawn from the tileresults in contraction of the two-phase structure (matrix andelectrolyte), subsequently resulting in the formation of gas pocketswhich contribute to gas crossover and high internal resistance.

Accordingly, what is needed in this art is a matrix material which isnot critically fragile, can withstand flexural and compressive forcesduring molten carbonate fuel cell assembly, and use, and can achieve asatisfactory inert particle distribution.

DISCLOSURE OF INVENTION

The present invention is directed to a matrix material for a moltencarbonate fuel cell which is thin, flexible, pliable and compliant atroom temperature. The matrix comprises a cast mixture of small particlesinert to molten carbonate electrolyte, larger inert ceramic particlesand an organic polymer binder. The matrix structure has a closely packedparticulate network having a uniform (preferably submicron) poredistribution.

Another aspect of the invention comprises a method of assembling amolten carbonate fuel cell utilizing the compliant matrix material. Thematrix material is inserted into the fuel cell stack provided with ananode of sufficient porosity to contain adequate electrolyte forcontinuous operation of a molten carbonate fuel cell for at least 40,000hours. The fuel cell stack containing such anode and matrix is heated toa temperature for a time sufficient to remove the polymer binder fromthe matrix and cause electrolyte material to wick into the matrix fromthe anode.

The foregoing, and other features and advantages of the presentinvention, will become more apparent from the following description.

BEST MODE FOR CARRYING OUT THE INVENTION

There are preferably three components in the tapes produced according tothe present invention. The first component is an inert particlepreferably less than about 1 micron in size. This produces a fine poresize (e.g. about 0.15 to about 0.3 micron average pore size) in theultimate matrix to insure electrolyte retention. γ lithium aluminate isthe preferred inert material, however, other material inert to themolten carbonate environment such as ceria, strontium titanate,strontium zirconate, etc. may be used.

The second component of the tape according to the present invention iscorrosion resistant ceramic particulate material having an average sizerange greater than about 25 microns and preferably greater than about 50microns in diameter. The function of this component is primarily crackattenuation. It can be made of the same material as the inert particlesabove, or a more reactive material such as alumina which, because of thelarger particle size has less surface area and, therefore, lessreactivity toward the molten carbonate electrolyte. This allows use ofceramic materials not generally considered compatible with a moltencarbonate system. Note commonly assigned U.S. patent application Ser.No. 158,019, filed June 9, 1980 now Pat. No. 4,322,482, the disclosureof which is incorporated by reference.

The third component is the temporary plastic binder. This binderprovides handleability, flexibility and conformability to the tape,three key structural properties. While any polymer which decomposes attemperatures lower than the molten carbonate fuel cell operatingtemperatures can be used, polyvinyl butyral (Monsanto Butvar B-98) ispreferred. Various plasticizers and other modifiers such as flow controlagents can be added to the polymer for application purposes.

The components are mixed together with an organic solvent and applied toa mold surface treated with a release agent. After drying, the thusformed tape is removed and is ready for assembly into the moltencarbonate fuel cell. The amounts of the materials may vary, but arepreferably used in a range of about 40% to about 45% by volume of theinert submicron particles, about 5% to about 30% and preferably about15% by volume of the larger, crack attenuating ceramic particles withthe balance being the plastic binder material. The materials arepreferably mixed in a ball mill with organic solvents such aschlorinated hydrocarbons and alcohol.

The main characteristics of the matrix tape according to the presentinvention as compared with the prior matrices are is its pliability andcompliance which, when placed between the electrodes in a moltencarbonate fuel cell, allow it to conform to the irregularities of suchsurfaces producing more intimate contact, thus eliminating spaces thatmight otherwise interfere with the required ion transfer.

After the matrix tape is produced, electrolyte is preloaded into theanode, which has sufficient pore volume to enable it to contain enoughelectrolyte for the life of the cell (e.g. 40,000 hours) includingenough electrolyte to fill the matrix. Such anodes are generallyproduced to contain about 50% to about 70% by volume porosity(preferably about 50% to about 55%) with about 30% to about 95% of thatporosity electrolyte filled (preferably about 95%). The cell is heatedup gradually to decompose and strip the plastic binder prior toelectrolyte melting, allowing the electrolyte to wick out of the anodeand into the matrix. Cell seals and cathodes may also be provided withelectrolyte from this same anode source.

Chlorinated hydrocarbons and alcohols have been found to be thepreferred organic solvents for proper drying and flow control of thematrix tape material during forming. Alcohols such as ethanol andbutanol mixed with chlorinated hydrocarbons such as perchloroethyleneand an anti-foaming agent have been found to provide viscosity and flowproperties of the matrix material for easy application.

The coatings can be applied to the mold surface by any method such asbrushing, spraying, etc. although use of conventional curtain coatingand doctor-blade casting is preferred. Note "Doctor-Blade Process" by J.C. Williams which teaches a method of formulating electronic ceramictapes through the use of a doctor-blade process (Treatise On MaterialsScience and Technology, Vol. 9, Ceramic Fabrication Processes, FranklinF. Y. Wang ed.).

In the casting operation, a glass mold surface is preferred, and while avariety of mold release agents such as Teflon®(duPont de Nemours & Co.,Inc.) and beeswax have been used, beeswax has been found to be easy toapply and long-lasting during a plurality of casting applications. Thebeeswax can be applied in solution in perchloroethylene with a cleancloth. Master Sheet Wax (The Kindt-Collins Co., Cleveland, Ohio) hasbeen found to be particularly suitable for this purpose. It isparticularly desirable to apply several layers of the matrix compositewith drying (preferably air drying) between each application beforeremoval from the mold surface.

Chlorinated hydrocarbons and alcohols have been found to be thepreferred organic solvents for proper drying and flow control of thematrix tape material during casting. Alcohols such as ethanol andbutanol mixed with chlorinated hydrocarbons such as perchloroethylenehave been found to provide suitable flow properties to the slurry. Aslurry viscosity in the range of 800-1200 cps is preferred for castinglayers, either by doctor-blade or curtain coater. Other materials areadded to aid mixing, casting, and final tape properties. A deflocculantsuch as crude menhaden fish-oil aids in dispersing the ceramicparticles. An anti-foaming agent like Dow Antifoam A aids in the escapeof trapped gas bubbles during drying. Plasticizers like MonsantoSanticizer #8 prevent embrittlement of the dried tape. The fish-oil alsocontributes to flexibility.

Entrapment of gas bubbles in the tape requires their removal beforedrying. To aid this, drying may be retarded by use of solvents withlower vapor pressure, like butanol, or more effectively, by holding thefreshly cast tape in an atmosphere saturated with solvent vapors.Typically, a 15 minute delay before drying will allow bubbles to escape.The use of an anti-foam agent aids the breaking and release of thebubbles. If the solvent vapor treatment is utilized to remove entrappedgas bubbles before drying, any of the above-cited chlorinatedhydrocarbon or alcohol solvents can be used, although the use of anazeotropic mixture of perchloroethylene and ethanol is preferred.Treatment time should be sufficient to remove the bubbles; in mostinstances, times of at least 5 minutes being required.

EXAMPLE

788 grams of perchloroethylene, 260 grams of secondary isobutanol, and36 grams of Dow-Corning Antifoam-A were mixed together with 1200 gramsof calcined (4 hours at 1300° F., 704° C.) jet-milled γLiAlO₂, andball-milled with alumina balls for 24 hours to thoroughly disperse theLiAlO₂. 150 grams of Monsanto Santicizer #8 (N-ethylortho andpara-toluenesulfonamide), 750 grams of denaturedethanol, and 275 gramsof Monsanto Butvar B-98 (polyvinyl butyral) were then added, and ballmilling was continued for 48 hours. The alumina milling balls were thenremoved, and 60 grams of crude menhaden fish-oil and 632 grams of 120grit Alundum-38 (Al₂ O₃) were added. Everything was then rolled withoutballs in the ball-mill to mix the ingredients without further grinding.The mill was then rotated slowly enough (1 to 2 rpm) to allow the escapeof most trapped air bubbles without allowing the alumina to settle out.This solution was applied with a doctor-blade to glass sheets treatedwith a beeswax mold release agent. Coatings 9-12 mils thick were appliedand allowed to air dry for 5-15 minutes to a thickness of about 5 to 6mils. This process was repeated until coatings 12-25 mils thick wereobtained. The final dried tape was easily removable from the moldsurface and had a compliant consistency. When measured on a Shore ADurometer a reading of 94 was typically seen. A ten-cell moltencarbonate fuel cell stack of one square foot sheets was next assembledby placing matrix tapes between porous nickel anodes prefilled withelectrolyte to approximately 95% of their porosity, and porous nickelcathodes (nickel oxide has also been used), with separator platesbetween cells and heating according to the following schedule. It shouldbe noted that while this example is described in terms of a ten-cellstack electrode-matrix assembly, an entire fuel cell stack can consistof approximately 500 of such electrode matrix assemblies which can beheat treated according to this process. The ten-cell stack of the abovedescribed cells was heated from room temperature up to 205° C. in an airenvironment with a temperature increase of 40° C. per hour. Once atemperature of 205° C. was attained, the stack was held there for 6hours. The temperature of the stack was next raised to 315° C. inincrements of 40° C. per hour and held there for 6 hours. This removesall solvent from the tape and volatilization of the polymer begins. Thetemperature of the stack was next raised again at 40° C. per hour to470° C. and held there for 2 hours. Complete pyrolization andvolatilization of the polymer now occurs. Following this, reducing gaswas introduced into the stack anode compartments and the temperatureraised above the melt point of the electrolyte (about 490° C.) at therate of 40° C. per hour until a stack operating temperature ofapproximately 650° C. was reached.

While the invention has primarily been described in terms of particularinert particles, ceramic particles, polymer binders, solvents andrelease agents, it would, of course, be within the purview of oneskilled in this art to use any materials which are compatible withmolten carbonate at fuel cell operating conditions.

Although this invention has been described with respect to detailedembodiments thereof, it will be understood by those skilled in the artthat various changes in form and detail thereof may be made withoutdeparting from the spirit and scope of the claimed invention.

We claim:
 1. In the process of assembling a molten carbonate fuel cell including forming a fuel cell stack by stacking a plurality of electrodes separated by layers of matrix material, wherein the improvement comprises: inserting as the matrix material a matrix comprising particles inert to molten carbonate electrolyte having a particle size less than about one micron, ceramic particles having a particle size greater than about 25 microns, and an organic polymer binder material, the binder material being present in an amount of at least about 35% by volume, the matrix material being flexible, pliable, and compliant at room temperature, into the fuel cell stack, and utilizing as the fuel cell anode, an anode with sufficient porosity to contain sufficient electrolyte for continuous operation of the molten carbonate fuel cell for at least 40,000 hours.
 2. The process of claim 1 including heating the fuel cell stack containing the matrix and anode to a temperature for a time sufficient to remove the polymer binder and cause at least a portion of the electrolyte material in the anode to wick into and substantially completely fill the matrix.
 3. The process of claim 2 wherein the stack is heated from room temperature up to 205° C. at a rate of 40° C. per hour, held at 205° C. for 6 hours, then heated to 315° C. at a rate of 40° C. per hour and held at 315° C. for 6 hours, and then heated to 470° C. at a rate of 40° C. per hour and held at 470° C. for 2 hours, before being raised above the electrolyte melt temperature to a stack operating temperature of approximately 650° C.
 4. The process of claim 1 wherein the inert particles are present in about 40% to about 45% by volume and the ceramic particles are present in an amount of at least about 15% by volume.
 5. The process of claim 1 wherein the polymer is polyvinyl butyral.
 6. The process of claim 1 wherein the inert material is lithium aluminate.
 7. The process of claim 1 wherein the ceramic particles are alumina. 